The present invention relates to a phototropic glass which in the unexposed condition is either tinted or colorless and which has a brown tint when exposed to actinic light.
Phototropic glasses (see for example German PS No. 15 96 847) go dark or change color under the influence of ultra-violet to visible radiation and lighten up again when the exciting radiation is withdrawn. The phototropic components contained in these glasses are mostly silver-alkali-metal-halide phases dispersed in the glass, which are doped, for example, with Cu. The basic glass composition is usually from the system R.sub.2 O--Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 --SiO.sub.2 and covers, in percent by weight, the composition range 40-76% SiO.sub.2, 4-26% B.sub.2 O.sub.3, 2-26% Al.sub.2 O.sub.3 and at least one of the alkali metal oxides 0-12% Li.sub.2 O, 0-10% Na.sub.2 O, 0-17% K.sub.2 O, 0-24% Rb.sub.2 O, the combined percentage of these components, including the silver halides, amounting to at least 85% of the total composition.
Transparent glasses should not contain more than 0.7% of silver or of the total of the three halides (Cl, Br,J); at the same time the quantity of the silver halide-containing precipitations should not exceed 0.1% by volume, and their discrete size should not exceed a diameter of 0.1 .mu.m.
Spectacle-lenses are the primary field of application for these phototropic glasses. A typical composition, which has for a long time been on the market under the trade name "Photosolar," contains the following oxides, in wt.%;
______________________________________ SiO.sub.2 55.9 B.sub.2 O.sub.3 15.9 Al.sub.2 O.sub.3 8.9 Li.sub.2 O 2.5 Na.sub.2 O 0.8 K.sub.2 O 0.1 BaO 7.5 MgO 2.3 CaO 0.2 PbO 4.2 ZrO.sub.2 1.0 Ag 0.16 Cu 0.035 Cl 0.24 Br 0.145 F 0.19 ______________________________________
In developing the composition of this glass, consideration was given to the requirements peculiar to its application as a glass for spectacle lenses taking into account its photographic as well as ophthalmic properties and also its suitability for chemical hardening. (Description of "Photosolar" see catalogue, Glas fur Korrektions- und Sonnenschutzbrillen" DESAG, 3223 Delligsen 2, POB 80, Fed. Republic of Germany.
In pursuit of a further improvement in these properties and particularly accelerating the process of photochromic darkening and recovery to original lightness, the following glass was developed:
______________________________________ SiO.sub.2 55.74 B.sub.2 O.sub.3 17.65 Al.sub.2 O.sub.3 6.40 Li.sub.2 O 1.92 Na.sub.2 O 3.72 K.sub.2 O 5.85 TiO.sub.2 2.23 ZrO.sub.2 5.00 CuO 0.008 Ag 0.28 Cl 0.26 Br 0.18 ______________________________________
See catalogue, Glas fur Korrektions- und Sonnenschutzbrillen DESAG, 3223 Delligsen 2, POB 80, Fed. Republic of Germany.
The critical factor for good phototropic properties is the molar ratio of the sum of the alkali metals content relative to that of the boric acid component which, if the glass is free of bivalent cations (except for Cu.sup.++), should be 0.55-0.85, and also the weight ratio Ag: (Cl+Br), which should be 0.6 to 0.9. This glass has been produced for some time under the trade name "Photosolar Super."
A still further improved glass was proposed in EP-OS 0063 790 which has the following composition:
______________________________________ SiO.sub.2 39.10 B.sub.2 O.sub.3 18.00 P.sub.2 O.sub.5 0.25 Al.sub.2 O.sub.3 0.10 ZrO.sub.2 5.50 La.sub.2 O.sub.3 0.50 WO.sub.3 1.00 CaO 0.10 SrO 1.00 PbO 15.55 TiO.sub.2 5.70 Li.sub.2 O 2.20 Na.sub.2 O 2.20 K.sub.2 O 9.00 Ag.sub.2 O 0.27 CuO 0.009 Cl 0.64 Br 1.21 ______________________________________
For better growth control of the silver halide-containing precipitations and for improved uniformity and repeatability of phototropic behavior, an initial vitrifying charge composition is melted down, the melt being rapidly cooled into a glass and molded, and the glass molding then being subjected to a heat treatment for the formation of nuclei or seeds and growth of the silver-alkali-halide precipitate.
The two last mentioned phototropic glasses darken to gray shades when exposed to light radiation.
Permanent tinting which is superimposed on the phototropism may be achieved in the usual manner. For example, various tints can be obtained by addition of rare earths such as Er.sub.2 O.sub.3, Pr.sub.2 O.sub.3, CeO.sub.2, Ho.sub.2 O.sub.3 or Nd.sub.2 O.sub.3 or of transition metals such as CoO, NiO, Mn.sub.2 O.sub.3 or Cr.sub.2 O.sub.3. German AS No. 17 71 063 describes tinted phototropic glasses of this type. However, all of these glasses are permanently tinted or colored, the glass remaining colored in the phototropically darkened state as well as in the recovered state.